In the yarn processing technique proximity sensors are extensively used for yarn detection or motion detection. EP-B-171 516 relates to a yarn feeder in which several proximity sensors scan the size of a yarn store built on the storage drum (minimum-, maximum- or reference-store size sensor) for controlling the drive motor of a winding on element. In this case the magnet, e.g. a permanent magnet, causing a Hall element of the proximity sensor to respond, is moved by the yarn store from a first position in which the magnet is closer to the Hall element and a second position in which it is further distant from said Hall element. The Hall element then is responding to said motion digitally or analogously. The proximity sensor generates a signal which controls the drive motor (to switch it on or off, to accelerate or decelerate it). Frequently in such a yarn feeder furthermore a proximity sensor is provided as a speed- or counting sensor responding to the passage of a magnet rotating with the drive shaft or the winding on element, respectively, by a signal. Such proximity sensors are extraordinarily reliable electronic components. For cost reasons in almost all cases a Hall element can be used, but also a magneto-resistive sensor element or an inductive sensor element, in order to gain signals from the relative motion of a body made from magnetic and/or electromagnetic flux conducting material. It has shown in practice that in a production series of yarn feeders or among the several proximity sensors in one and the same yarn feeder the response sensitivity of the proximity sensors does fluctuate. Supposedly this is due to the properties of the proximity sensors and occasionally also to manufacturing or assembly tolerances. For example, in one proximity sensor used as a maximum store size sensor the switching point is situated at another distance between the Hall element and the magnet than at the same maximum store size sensor of a further yarn feeder of the same production series. This even can occur among the proximity sensors in one and the same yarn feeder. It is desirable to achieve among the proximity sensors in one and the same yarn feeder the same response behavior, or to guarantee among equal proximity sensors in different yarn feeders of a production series the same response behavior such that motor control is carried out identically. When using proximity sensors having a Hall element it is generally known in practice to improve the response of said Hall element by means of a metal body secured in the vicinity of said Hall element to concentrate the magnetic flux.
A Reed switching device for a safety monitoring system or a machine control system is known from U.S. Pat. No. 4,213,110 according to which a preloading solenoid is associated to an encapsulated Reed switch. The preloading solenoid can be shifted parallel to the longitudinal direction of said Reed switch in order to change the sensitivity of said Reed switch.
From U.S. Pat. No. 5,172,052 a current sensor is known for scanning contactlessly the current flowing in conductors. Between the conductors of a pair of conductors a Hall element is provided having the shape of a flat square prism. In order to concentrate or direct the magnetic flux an adjustment screw is associated to said Hall element. By adjusting the adjustment screw in its longitudinal direction the flux density can be increased or decreased in the Hall element in order to adjust the sensitivity of the current sensor.